Galaxies merged together house the greediest voids

Supermassive black holes in merger galaxies snack on one star every year, according to a paper published on Thursday.

The Andromeda galaxy, also known as Messier 31 or M31, sits next door to the Milky Way only about 2.5 million light years away. It’s a giant spiral galaxy teeming with stars and astroboffins have spotted a pair of black holes tightly locked together in close orbit.

Scientists in the US have estimated that the total mass of both black holes is about two hundred million times the mass of the Sun. It is believed that the one black hole originated from another galaxy, but they both ended up near each other due to a merger event.

The collision of galaxies can sometimes breed particularly greedy black holes like this pair. Heather Wernke, coauthor of the study published in The Astrophysical Journal, and a graduate student at the University of Colorado Boulder (UC Boulder), said: "We predict that in a post-galactic merger period, a supermassive black hole will swallow one star per year. That's 10,000 times more often than other rate predictions."

The high stellar-munch rate is due to the complex gravitational effects tugging between both black holes. Surrounding stars are pulled and arranged around them in an eccentric disk, and their orbits are stretched. As the stars revolve around the supermassive black holes, they can interact with one another and sometimes their orbits overlap.

Gravitational disruptions can jog a star’s orbit and fling it closer to the black hole. If it gets too close, however, it’ll be swallowed.

Shredded

Ann-Marie Madigan, first author of the paper and an assistant professor working in the department of astrophysical and planetary sciences at UC Boulder, explained that "the force builds up in these stellar orbits and changes their shape. Eventually, a star reaches its nearest approach to the black hole and it gets shredded."

By examining the stability of the eccentric disk, the team estimate that supermassive black holes like the ones found in Andromeda can ingest a star every year. Over time, this rate drops as the number of stars in the disk get gobbled up.

"Andromeda is likely past the peak of this process, having undergone a merger long ago. But with higher resolution data, we may be able to find younger eccentric disks in more distant galactic nuclei," Madigan said.

The study could be important in studying how these supermassive black holes develop she explained to The Register. It also answers "a long-standing question about why these eccentric disks are stable, since the same gravitational forces that send stars onto dangerously eccentric orbits also holds the disk together."

"[We are] proposing that these disks directly cause huge numbers of stars to be eaten by black holes. The latter could be important for the growth of supermassive black holes."

If humans are still around in four billion years they could test out this theory, since Andromeda is on a collision course with our own Milky Way. It's currently heading our way at 110 kilometres per second, or 3.669 per cent of the maximum velocity of a sheep in a vacuum in Reg units. ®